U.S. patent application number 13/709694 was filed with the patent office on 2013-06-13 for quiet wind turbine blade.
This patent application is currently assigned to Inventus Holdings, LLC. The applicant listed for this patent is Inventus Holdings, LLC. Invention is credited to Daniel Brake, Gregory J. Smith.
Application Number | 20130149162 13/709694 |
Document ID | / |
Family ID | 48572143 |
Filed Date | 2013-06-13 |
United States Patent
Application |
20130149162 |
Kind Code |
A1 |
Smith; Gregory J. ; et
al. |
June 13, 2013 |
QUIET WIND TURBINE BLADE
Abstract
Noise produced by the blades of a wind turbine electric
generator is reduced by creating randomly varied serrations on the
trailing edge of the blade, thereby creating vortices of varying
size, energy, and distance from the trailing edge. This
configuration creates vortices that tend to cancel one another
while avoiding the creation of adjacent, mutually reinforcing
vortices.
Inventors: |
Smith; Gregory J.; (Port St.
Lucie, FL) ; Brake; Daniel; (Hobe Sound, FL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Inventus Holdings, LLC; |
Juno Beach |
FL |
US |
|
|
Assignee: |
Inventus Holdings, LLC
Juno Beach
FL
|
Family ID: |
48572143 |
Appl. No.: |
13/709694 |
Filed: |
December 10, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61568302 |
Dec 8, 2011 |
|
|
|
Current U.S.
Class: |
416/223R |
Current CPC
Class: |
F03D 1/0633 20130101;
F05B 2250/183 20130101; F05B 2240/122 20130101; Y02E 10/72
20130101; Y02E 10/721 20130101; F05B 2260/96 20130101 |
Class at
Publication: |
416/223.R |
International
Class: |
F03D 1/06 20060101
F03D001/06 |
Claims
1. A blade of a wind turbine comprising: an airfoil, said airfoil
having a substantially straight trailing edge and a plurality of
essentially planar serrations located at said trailing edge; each
of said plurality of serrations comprising two or more
substantially straight edges such that edges of adjacent serrations
meet to form a trough comprising an angle, said trough defining a
trough length as being the shortest distance between said trough
and the nearest point of said trailing edge; each of said plurality
of serrations terminating at a point comprising an angle, each said
point being at the farthest downwind extremity of each said
serration, each said point defining a serration length comprising a
line extending from said point to the nearest point of said
trailing edge; said trough lengths being substantially equal and
said serration length lines being substantially equal and bisecting
said point angles into substantially equal angles, such that no
three adjacent points are equally spaced apart from each other.
2. A blade of a wind turbine comprising: an airfoil, said airfoil
having a substantially straight trailing edge and a plurality of
essentially planar serrations located at said trailing edge; each
of said plurality of serrations comprising two or more
substantially straight edges such that edges of adjacent serrations
meet to form a trough comprising an angle, said trough defining a
trough length as being the shortest distance between said trough
and the nearest point of said trailing edge; each of said plurality
of serrations terminating at a point comprising an angle, each said
point being at the farthest downwind extremity of each said
serration, each said point defining a serration length comprising a
line extending from said point to the nearest point of said
trailing edge; said trough lengths being substantially equal and
said serration length lines bisecting said point angles into
substantially equal angles, such that no three adjacent length
lines are equal in length.
3. A blade of a wind turbine comprising: an airfoil, said airfoil
having a substantially straight trailing edge and a plurality of
essentially planar serrations located at said trailing edge; each
of said plurality of serrations comprising two or more
substantially straight edges such that edges of adjacent serrations
meet to form a trough comprising an angle, said trough defining a
trough length as being the shortest distance between said trough
and the nearest point of said trailing edge; each of said plurality
of serrations terminating at a point comprising an angle, each said
point being at the farthest downwind extremity of each said
serration, each said point defining a serration length comprising a
line extending from said point to the nearest point of said
trailing edge; said serration length lines being of substantially
equal length and no three trough lengths being equal.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/568,302 filed Dec. 8, 2011, the disclosure of
which is hereby incorporated herein by reference in its
entirety.
BACKGROUND
[0002] Wind turbine technology is providing an ever-increasing
proportion of the electrical energy generated in the United States
and worldwide. Typically, at least in the United States, wind farms
have been located in rural or sparsely populated areas. However,
with an increasing demand for the renewable energy represented by
wind turbine generators, it is likely that wind farms will in the
future be situated closer to populated areas than they are today.
When that happens, nuisances created by wind farms will become
problematical as people discover that there are esthetic
impediments and practical drawbacks to living or working in close
proximity to a wind farm. Chief amongst these is the fact that wind
turbines are noisy.
[0003] Studies conducted to determine both the level of noise
generated by wind turbines, and the frequencies and pervasiveness
of such noise, provide startling conclusions that wind turbine
noise is a significantly greater annoyance to humans than
previously suspected. Utility wind turbine generators, for example,
can develop noise in the range of 90-100 db at or near the base of
the tower. While some of the noise of a wind turbine generator is
based upon mechanical factors such as gear boxes, generators, and
turning parts, a significant portion of the noise is due to
aerodynamic factors related to the shape of the blades that catch
the wind.
[0004] Wind turbine blades are airfoils that provide lift by means
of the differential in pressure as air flows over the blades. The
differential pressure and subsequent lift is caused because air
passing over the top of the blade travels faster than air traveling
over the bottom of the blade. As the air converges at the trailing
edge of the blade the differing air velocities cause vortices.
Vortices are whirling air disturbances that maintain their general
shape until the energy within them dissipates, at which point they
collapse, resulting in an implosion of air that raises sound
pressure levels, or noise. Regular vortices that form at the
trailing edge of an aerodynamic surface such as, for example, a
helicopter main rotor blade, create distinctive sounds as they
collapse. The same phenomenon causes wind turbine generators to
make noise as the blades rotate in response to wind. Where the
trailing edge of an airfoil is substantially straight, vortices of
approximately equal strength will develop as air passes over the
airfoil, and will tend to collapse substantially simultaneously at
a given distance behind the trailing edge. The sound made by the
collapse of individual vortices is reinforced through the
simultaneous collapse of adjacent vortices created by the same
trailing edge, with the result that appreciable and potentially
annoying noise is generated behind each blade of a rotating wind
turbine.
[0005] A number of techniques have been employed to reduce or
change the pattern of vortices caused by air confluence at trailing
edge of the blade, and thereby reduce noise emissions. One such
method is to add serrations to the trailing edge of the blade in
order to reduce the vortices thereby reducing the sound pressure
level. The serrations are generally equally spaced serrations of
the same size positioned along the trailing edge of the blade. See,
e.g., U.S. Pub. No. 2009/0074585 to Koegler et al.; U.S. Pat. No.
7,059,833 to Stiesdal et al.
[0006] The major problem with the current technology however is
that although serrations may be effective in reducing overall noise
levels, the resulting frequency and amplitude of vibration remain
as a constant repeating pattern. What is needed is a means to
reduce aerodynamic noise generated by commercial wind turbine
blades without significantly reducing blade efficiency or adding to
the weight or cost of the unit.
SUMMARY OF THE INVENTION
[0007] The invention is to create randomly varied serrations on the
trailing edge of the blade of a wind turbine. By so doing, vortices
of varying size and energy are created as the blade turns and,
because the vortices are random and irregular, they will tend to
cancel or negate one another, rather than reinforce the adjacent
vortices with which they interact. The result is to create vortices
of randomly variable energies, frequencies and amplitudes that will
not implode in regular patterns that cause noise.
[0008] Variations in the serration pattern along the trailing edge
of a blade may take such forms as points of differing lengths
(measured along a perpendicular line from the trailing edge of the
blade to the tip of the serration), differing angles at the points
of adjacent serrations, and differing angles between the trailing
edge of the blade and lines bisecting the angles formed at the
point of each serration. These varying serration parameters mimic
some of the random feather patterns noted in the wings of birds of
prey, whose ability to capture prey depends upon essentially
noiseless flight, and who may be distinguished from other bird
species whose wings do not exhibit that characteristic.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 depicts a conventional serrated trailing edge of the
kind used in the prior art.
[0010] FIG. 2 depicts a second variation of serrated trailing edge
that is used in the prior art.
[0011] FIG. 3 shows an embodiment of the serrated trailing edge of
this invention.
[0012] FIG. 4 illustrates a second embodiment of the serrated
trailing edge of this invention.
[0013] FIG. 5 shows a third embodiment of the serrated trailing
edge of this invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0014] Prior art attempts to alleviate the problem of noise
associated with the rotation of turbine blades have met with mixed
results. FIG. 1, depicts a prior art configuration that allows the
formation of regular vortices at the trailing edge of an airfoil,
such as an aircraft wing or a helicopter or turbine blade. The
regular and repeated pattern of FIG. 1 permits air currents to flow
directly to the rear of the airfoil, as indicated by arrows 10. The
trailing edge has serrations characterized by peaks or points 20
and troughs or valleys 30. In the regular patters shown in FIG. 1,
all of the troughs terminate along a line A-A incident or parallel
to the trailing edge of the airfoil. All of the peaks terminate
along a line B-B that is substantially parallel to line A-A and the
trailing edge of the airfoil. Vortices 40 (indicated by dotted
lines) develop at the points where the airflow 10 leaves the
trailing edge of the airfoil. Such vortices comprise high energy
twisting airstreams having a characteristic circular cyclonic or
anti-cyclonic motion and capturing an area of relatively lower
pressure within the center region. As the vortices travel
downstream relative to the trailing edge of the airfoil, their
rotational energy causes the circular currents to expand,
increasing their radii and dissipating energy. At a point
downstream of the trailing edge, adjacent vortices may interact,
the point of interaction being designated by heavy, non-broken
lines at reference numeral 50. Where the vortices interact, chaotic
motion may occur, and the energy that powered the circular
rotational motion is suddenly dissipated. At that point, the
vortices may collapse in to the lower pressure region, causing
audible noise that, repeated along the length of the airfoil,
provides the characteristic sound associated with the particular
airfoil. In the case of wind turbine blades, the characteristic
whoosh-whoosh-whoosh sound produced as each blade passes a
particular point is easily recognizable. An extreme example of such
noise is that which is heard when a helicopter is lifting off or
operating under a heavy load.
[0015] In the configuration shown in FIG. 1, vortices formed at the
points 20 of the serrated trailing edge form regular patterns such
that noise generated when the vortices collapse is repeated and
reinforced along the length of the airfoil. When this trailing edge
is applied to a wind turbine blade, the regular repeating pattern
depicted in FIG. 1 fails to suppress the noise generated by the
blade and may, in some instances, actually amplify it.
[0016] One prior art attempt at alleviating the problems indicated
by FIG. 1 is depicted in FIG. 2. In FIG. 2, serrations indicated by
points 20 and troughs 30 have been skewed to form triangular points
that do not form repeating angles with respect to the trailing
edge, indicated by line A-A. However, the critical points that
affect the formation of vortices--the points and troughs of the
serrations--remain essentially parallel, meaning that vortex
formation will occur at essentially the same distance downstream of
the trailing edge of the airfoil for all vortices. In the absence
of vortex-to-vortex interference, it may be expected that the
vortices will each have approximately the same energy, and may
dissipate that energy at the same rate, with the result that vortex
collapse would occur substantially simultaneously for all vortices,
and the noise of each vortex collapse would be reinforced and
amplified by others occurring at the same time.
[0017] In this configuration, however, air currents leaving the
trailing edge of a wing or blade may not be parallel because of the
irregular angles at which adjacent serrations meet. The resulting
vortex formation may exhibit characteristics of being skewed in a
cross-wind direction, and may generate areas of higher and lower
pressure immediately behind the blade. At subsonic speeds, this
phenomenon may affect upstream airflow across the blade, and could
result in reduced aerodynamic performance of the blade. Thus, while
regular vortex formation can be avoided, the chaotic and
unpredictable nature of vortices created by this configuration may
result in reduced efficiency and a loss of power across the
operating spectrum.
[0018] FIG. 3 depicts one embodiment of the present invention. In
FIG. 3, adjacent vortices 40 are spaced apart at irregular
intervals, and the angles forming adjacent troughs 30 and points 40
are similarly irregular and non-repeating. In this case, the points
of intersection 50 of adjacent vortices is varied with respect to
the distance from the trailing edge of the blade. If the vortices
have sufficient energy to avoid collapse until they begin to
interact with adjacent vortices, the point at which the vortices
interact and collapse will occur at different distances behind the
trailing edge of the blade. When this happens, the noise associated
with the collapse will occur at slightly different times, and the
reinforcing effect of simultaneous collapses may be avoided,
resulting in a lower noise level.
[0019] A similar result may be achieved with the configuration
depicted in FIG. 4. In FIG. 4, the points 20 of adjacent serrations
have been varied with respect to the trailing edge A-A, as can be
seen in adjacent points ending at lines B-B, B'-B', and B''-B''.
The advantageous effect of varying the point-to-trailing edge
distance for adjacent points 20 may be further enhanced where the
spanwise distance between adjacent points 20 is also varied such
that the points of intersection 50 of a series of adjacent vortices
40 occur upstream or downstream with respect to one another and
with respect to the trailing edge A-A.
[0020] FIG. 5 illustrates yet another embodiment for a serrated
trailing edge of a turbine blade. In FIG. 5, the troughs terminate
at varying distances from the trailing edge, as can be noted by
lines A-A, A'-A', and A''-A'', each of which demarks the
termination point of a trough. The effect is similar to that
achieved by the configuration of FIG. 4.
[0021] The configurations depicted in FIGS. 4 and 5 allow the
manipulation of vortex formation to be controlled and predicted.
Varying angles between troughs 30 and points 40 may be expected to
produce vortices of differing initial sizes and energies, further
depending upon wind velocity across the blade and pressure
differences between the high and low-pressure sides of the blade.
In other embodiments, both the trough-to-trailing edge and the
point-to-trailing edge distances may be varied, as well as the
point-to-point spanwise distance, to control the points at which
vortices may form, and the initial sizes and energy for them. In
yet other embodiments, the points may also be varied in a zig-zag
pattern to terminate above or below the plane formed by the
trailing edge.
[0022] Serrated strips may be retrofitted to existing wind turbine
installations by forming the serrations on suitable metallic,
fiberglass, plastic, or other suitable material and affixing the
strips to the trailing edges of turbine blades. Preferably,
serrated strips should be applied to the outer portions of a blade,
and should be mounted essentially along and parallel to the plane
of the low pressure side of the blade near the trailing edge.
* * * * *